In magnetic recording disk drives the magnetic material (or media) for the recording layer on the disk is chosen to have sufficient coercivity such that the magnetized data regions that define the data “bits” are written precisely and retain their magnetization state until written over by new data bits. As the areal data density (the number of bits that can be recorded on a unit surface area of the disk) increases, the magnetic grains that make up the data bits can be so small that they can be demagnetized simply from thermal instability or agitation within the magnetized bit (the so-called “superparamagnetic” effect). To avoid thermal instabilities of the stored magnetization, media with high magneto-crystalline anisotropy (Ku) are required. The thermal stability of a magnetic grain is to a large extent determined by KuV, where V is the volume of the magnetic grain. Thus a recording layer with a high Ku is important for thermal stability. However, increasing Ku also increases the short-time switching field H0 of the media, which is the field required to reverse the magnetization direction. For most magnetic materials H0 is substantially greater, for example about 1.5 to 2 times greater, than the coercive field or coercivity Hc measured on much longer time-scales. Obviously, the switching field cannot exceed the write field capability of the recording head, which currently is limited to about 12 kOe for perpendicular recording.
Since it is known that the coercivity of the magnetic material of the recording layer is temperature dependent, one proposed solution to the thermal stability problem is thermally-assisted recording (TAR), also called heat-assisted magnetic recording (HAMR), wherein the magnetic recording material is heated locally during writing to lower the coercivity enough for writing to occur, but where the coercivity/anisotropy is high enough for thermal stability of the recorded bits at the ambient temperature of the disk drive (i.e., the normal operating or “room” temperature of approximately 15-30° C.). In some proposed TAR systems, the magnetic recording material is heated to near or above its Curie temperature. The recorded data is then read back at ambient temperature by a conventional magnetoresistive (MR) read head.
Several TAR approaches have been proposed. TAR disk drives with a “small-area” heater direct heat to just the area of the data track where data is to be written by the write head. The most common type of small-area TAR disk drive uses a laser source and an optical waveguide with a near-field transducer (NFT). A “near-field” transducer refers to “near-field optics”, wherein the passage of light is through an element with subwavelength features and the light is coupled to a second element, such as a substrate like a magnetic recording medium, located a subwavelength distance from the first element. NFTs typically use a low-loss metal (e.g., Au, Ag, Al or Cu) shaped in such a way to concentrate surface charge motion at a surface feature shaped as a primary apex or tip. Oscillating tip charge creates an intense near-field pattern. The electromagnetic field of the oscillating tip charge gives rise to optical output in the near field, which is directed onto to the magnetic recording medium to heat just the area exposed to the write field from the write head. Small-area heaters have the advantage that they do not cause adjacent-track erasure (ATE). If data tracks adjacent to the data track being written were to also be heated, the stray magnetic field from the write head may erase data previously recorded in the adjacent tracks. Also, even in the absence of a magnetic field, the heating of adjacent data tracks will accelerate the thermal decay rate over that at ambient temperature and thus data loss may occur. While providing the advantage of less ATE, small-area heaters are difficult to fabricate. Also, because of the relatively inefficient heat transfer the heater temperatures required to reach a desired media temperature are very high.
TAR disk drives with a “wide-area” heater that heat an area much wider than the data track were proposed prior to the proposal for “small-area” heaters. A wide-area heater, typically a waveguide coupled to a laser and with an output end near the media, is relatively easier to fabricate and implement in a conventional recording head structure than a small-area heater. However, wide-area heaters have been shown to result in substantial ATE if the peak temperature extends into the adjacent tracks. Additionally, the waveguide needs to be located very close to where writing occurs, i.e., the trailing edge of the write pole, so that there is a minimal temperature drop from the time the media is heated to its peak temperature to the time it is exposed to the write field.
Disk drives that use “shingled recording”, also called “shingled-writing”, have also been proposed. In shingled recording, the write head, which is wider than the read head in the cross-track direction, writes magnetic transitions by making a plurality of consecutive circular paths that partially overlap. The non-overlapped portions of adjacent paths form the data tracks, which are thus narrower than the width of the write head. The data is read back by the narrower read head. When data is to be rewritten, all of the data tracks in an annular band are also rewritten.
The previously-cited related application discloses a TAR disk drive with a “wide-area” heater that also uses shingled recording and avoids the problem of ATE.
What is needed is a TAR disk drive that uses shingled recording and that has an optical waveguide that minimizes the temperature drop between the peak temperature in the media and the temperature in the media at the trailing edge of the write pole, while keeping the required laser power low.